Naturally occurring ores, particularly ores that contain metallic oxides, are routinely mined, purified and used in diverse applications. For example, ores that contain titanium oxides may be mined and purified to generate titanium dioxide that is suitable for use in paper, plastics and coatings applications, as well as in applications such as sunscreens and photocatalysts.
The two most common processes for purifying ores such as ores that contain titanium oxides are the chloride process and the sulfate process. Both of these processes are well known to persons skilled in the art. The chloride process, which is the more widely used of these two processes, requires reacting titania ore with gaseous chlorine to form titanium tetrachloride, then cooling and purifying the titanium tetrachloride to remove impurities, such as impurity chlorides and other noxious substances that are present in the chlorinated ore.
After one removes the impurity chlorides and other noxious substances, the purified titanium tetrachloride is oxidized to produce a base pigmentary particle of titanium dioxide. This base pigmentary particle of titanium dioxide may be further processed and/or treated to make a final pigment product and used in the aforementioned applications.
Through the chloride process, each year significant amounts of commercially acceptable titanium dioxide are generated around the world. However, as noted above, when generating commercially acceptable titanium dioxide, impurity chlorides and other noxious substances are also generated.
Among the noxious substances that are generated during chloride processes are dioxins, furans, benzo furans and other similar compounds. These substances are collectively referred to herein as “furans.” Examples of furans include, but are not limited to compounds that comprise: (i) the total PCDD, which include 2,3,7,8-TCDD; 1,2,3,7,8-PeCDD; 1,2,3,4,7,8-HxCDD; 1,2,3,6,7,8-HxCDD; 1,2,3,7,8,9-HxCDD; 1,2,3,4,6,7,8-HpCDD; and 1,2,3,4,5,7,8,9-OCDD; as well as (ii) the total PCDF, which include 2,3,7,8-TCDF; 1,2,3,7,8-PeCDF; 2,3,4,7,8-PeCDF; 1,2,3,4,7,8-HxCDF; 1,2,3,6,7,8-HxCDF; 2,3,4,6,7,8-HxCDF; 1,2,3,7,8,9-HxCDF; 1,2,3,4,6,7,8-HpCDF; 1,2,3,4,7,8,9-HpCDF and 1,2,3,4,6,7,8,9-OCDF. The literature describes these compounds as being particularly dangerous because they are carcinogens and mutagens.
Of these substances, the dioxin, 2,3,7,8-TCDD (2,3,7,8-tetrachlorodibenzo-p dioxin) is the most infamous. It is comprised of chlorine and organic materials and is more commonly known as the toxic component of Agent Orange.
In the chloride process, the formation of furans occurs during the cooling of the chlorinator reaction gas. Unfortunately, for a number of reasons, the conditions under which this cooling takes place are particularly conducive to the formation of furans.
First, the presence of coke, fly ash, finely divided solids and chlorine facilitates furan formation.
Second, the process for cooling chlorinator reaction gases exposes the precursors of furans to a temperature that is conducive to furan formation for a relatively long period of time. In this process, gas exiting a chlorinator is usually at a temperature in the range of approximately 800° C. to 1000° C. However, the components of this gas must be cooled to a sufficiently low temperature before the impurities can be separated from the titanium tetrachloride.
Typically, the chlorination gas must be cooled by several hundred degrees. For example, the temperature may need to be reduced to a temperature that is desirable for use in a downstream separating apparatus, e.g., when using a cyclone, it may be desirable to operate in a temperature range of approximately 260°–265° C. Unfortunately, furan formation is most significant in the temperature range of between 300° C. and 400° C., a temperature range through which the chlorinator reaction gas must pass when being cooled to the temperature at which separation is possible.
Moreover, the more commonly used methods for cooling chlorination reactor gases, which include using a spray drier or crudely atomized titanium tetrachloride sprayed directly into a hot conveying duct, permit the chlorinator precursors to remain in the temperature range of 300° C.–400° C. for a relatively long amount of time. As persons skilled in the art are aware, a spray drier has a relatively slow rate of cooling that allows for the formation of furans when the precursors to the furans pass through the above-described formation temperature zone.
Similarly, in methods that use direct spraying into the conveying duct, the rate of cooling of the gas stream depends on the degree of atomization of the crude titanium tetrachloride, and due to the nature of crude titanium tetrachloride, which contains impurity solids, such as ore and coke fines, it is not practical to design a spray that discharges small enough droplets to cool the gas stream rapidly enough to prevent furan formation effectively. Thus, these cooling technologies slow the rate of cooling, which leaves the precursors exposed to furan forming temperatures for an extended time.
Third, known cooling processes facilitate the build up of metal chlorides and other impurities on the walls of the outlet ducts, thereby narrowing the ducts. This build up increases the generation of the furans in two ways: (a) it causes a certain amount of the precursors of the furans to remain in the duct for an extended period, thereby extending their time being subjected to the furan formation conditions; and (b) it narrows the lumen of the duct so that the precursors that are not affixed to the walls of the duct nonetheless travel through the duct more slowly.
Each of the aforementioned practicalities of the chloride process contributes to the formation of furans during industrial chloride processes. Unfortunately, to date there has been no satisfactory way to avoid these problems.
Recently governmental agencies around the world that are charged with regulating environmental issues have begun to focus their attention on the emission of furans. They have reduced the level of acceptable emissions, and are contemplating reducing them further. Thus, the issue of furan formation is becoming increasingly significant for commercial producers of titanium dioxide.
Accordingly, it would be desirable to provide cost-effective means for reducing furan emissions. The present invention addresses this problem.